It is proposed to develop a reaction for the specific labeling of those proteins on the outer surfaces of membranes and by the use of this reaction to characterize the surface protein differences of normal and virus transformed cells. The reaction is the reduction with sodium borotritide of a Schiff base which has been formed with pyridoxal phosphate and the N-terminal amino groups of proteins. The reaction is simple, specific, and sensitive. Once the proper conditions have been established for maximal incorporation of radioactive label with minimal amount of distortion of the membrane structure, the surface proteins of chick embryo fibroblasts and chick embryo fibroblasts transformed by Rous sarcoma virus will be compared. This will be done by polyacrylamide gel electrophoresis of the labeled membrane proteins. Cells infected with temperature sensitive non-transforming mutants of RSV as well as cells infected with avian leukosis viruses will also be analyzed. Attempts will be made to correlate any differences in the surface proteins to the transformed state. Finally, the proteins on the outer surface of the Rous sarcoma virus will be identified by this reaction.